1. Technical Field
The present disclosure relates to a communication apparatus, a method for data communication, and a recording medium storing a data communication program.
2. Description of the Background Art
Transmission procedures for facsimiles are defined by ITU (International Telecommunication Union) recommendation T.30 (Procedures for document facsimile transmission in the general switched telephone network). V.34, a fast (up to 33.6 kbp/s) MODEM recommendation (ISU-series) is established in 1994, and a transmission procedure for facsimile conforming to V.34 is defined in Annex F of T.30.
According to T.30, when the Recommendation V.34 is used for facsimile transmission, an error-correcting procedure is deemed essential and is defined in Recommendation T.4. In V.34-standard half-duplex facsimile communication, communication interaction is performed in Phase 1, channel (line) probing is performed in Phase 2, and Equalizer training is performed in Phase 3. More specifically, in Phase 1, the communication standard is defined. In Phase 2, symbol rate, carrier frequency, and pre-emphasis filter are defined. In Phase 3, equalizer is trained by primary channel training, and signal speed of the primary channel is defined at a reception side based on the training result. Then, the control channel and primary channel are alternately repeated.
The control channel includes control data relating to transmitted and received data, for example, image-related data (e.g., line density), signal speed, and whether or not the image has been properly received. The primary channel includes an S signal for re-synchronization, phase reversal signal Sbar, equalizer training signal PP, binary signal B1, and image data. A data format of the image data when the error-correcting procedure is used is defined in Recommendation T.4. The image data includes frames composed of 256 octets or 64 octets, and a maximum of 256 frames can be transmitted by the primary channel at one time.
In Phase 2, an INFO0c sequence signal and an INFO0a sequence signal are exchanged between a call MODEM at the transmission side and an answer MODEM at the reception side. The transmission side transmits a Tone B, and the reception side transmits a Tone A and a Tone Ā (Tone A phase reversal). After 40 m sec has elapsed, the transmission side transmits a Tone B (Tone B phase reversal) and line probing signals (probing Tones) L1 and L2 followed by a second Tone B in response to a second Tone A and an INFOh sequence signal from the reception side. Thus, the line characteristics that determine image data communication speed can be measured.
One example of the recovery procedure in Phase 2 is proposed in JP-H11-150580-A. In this example, when a call MODEM does not receive second Tone A from an answer MODEM, or the answer MODEM does not receive the second Tone B from the call MODEM, within a predetermined period (two seconds) normally, and furthermore the Tone B or the Tone A is detected within a certain period of time, the communication procedure conforming to the V.34 recommendation retunes to Phase 2.
As described above, the data communication procedure using telephone lines is defined by the ITU, and the MODEM that operates at a data signal speed of up to 33.6 kbps is defined in the V.34 recommendation. In addition, in the V.34 recommendation, training for Phase 1 to Phase 4 is executed as for preparing data communication. In the half-duplex operation of Phase 2, both call MODEM and answer MODEM exchange MODEM capabilities (the supported symbol rate, carrier frequency, and whether or not there is a power protection capability) through INFO0c/INFO0a. Then, both call MODEM and answer MODEM analyze line conditions by line probing (L1/L2), select symbol rate, carrier frequency, and reemphasis coefficient, and the answer MODEM transmits the result to the call MODEM as the INFOh signal again. It should be noted that all INFO sequences (INFO0a, INFO0c, and INFOh) are transmitted by using binary differential phase shift keying (DPSK) at 600 bps.
Herein, because the INFO0c/INFO0a signal is transmitted from the top of Phase 2, when the line is severely deteriorated, the INFO sequences cannot be received correctly, which is a problem. In addition, in communication using an internet protocol (IP) network, due to problems such as packet loss, the reception data may be missed, which is also a problem.
The above-described example procedure proposes the recovery procedure not for detection of the INFO0c and the INFO0a but for Tone detection, and therefore cannot solve the above-described problems corresponding to the INFO0c and the INFO0a.
In the V.34 recommendation, in anticipation of these problems, a recovery procedure is defined: When the INFO0c/INFO0a sequences cannot be received, they are transmitted again. However, when the INFO sequence is transmitted again in the recovery procedure, communication is stopped (error/end) because the procedure cannot proceed to the next step.